Many types of heat exchangers are commercially available on the market today. One of the primary problems with such heat exchangers, however, is that the tube bundle is normally fixed in the main housing of the heat exchanger so that it cannot be removed to easily repair any of the tubes in the tube bundle that may deteriorate. This means that, in order to repair one of these prior art heat exchangers, it is necessary to either remove the entire heat exchanger including the housing from its installation so as to gain access to the tube bundle in the heat exchanger to repair same or for a workman to enter the main housing and repair the defective tube while the heat exchanger is in place. This results in considerable down time of the equipment on which the heat exchanger is being used and further makes the repair of the particular tube that is deteriorated in the tube bundle extremely difficult since it cannot be removed from the housing to gain access thereto. If another heat exchanger is actually inserted into the installation while the deteriorated heat exchanger is being repaired, the cost of maintaining these spare complete heat exchangers is prohibitive from a maintenance standpoint.
One common application for heat exchangers is an economizer which preheats feedwater supplied to a steam boiler. One of the problems encountered with the use of such economizers is that corrosive condensates are formed if the average temperature of the flue gases from the boiler drops below the dew point of the particular flue gases produced by the boiler. These prior art economizers have solved the condensation problem by maintaining all surfaces in the economizer with which the flue gases come into contact above the condensation temperature of the flue gases. While this prior art technique has prevented condensation, it has at the same time limited the rate at which heat can be transferred from the flue gases to the feedwater passing through the heat exchanger in the economizer. This is because this technique lowers the difference between the temperature of the flue gases and the temperature of the feedwater in the economizer which in turn controls the rate of heat transfer from the flue gases to the feedwater in the economizer. The net result has been that these prior art economizers required large heat exchangers therein in order to maximize the heat recovered from the flue gases by the feedwater passing through the economizer.